1. Technical Field
The present invention relates to video multicasting and, in particular, to scheduling video multicast transmissions in wireless orthogonal frequency division multiplexing networks.
2. Description of the Related Art
Orthogonal frequency division multiple access (OFDMA) has become the preferred air interface technology for next-generation broadband access networks such as 3GPP (Third Generation Partnership Project), LTE (Long Term Evolution), and WiMAX (Worldwide Interoperability for Microwave Access), including both macrocell and femtocell networks. Further, the proliferation of mobile devices has increased the demand for bandwidth intensive video applications and services such as mobile IPTV (Internet Protocol Television) and Video-on-Demand, where multicasting forms an important component.
In OFDMA, each sub-channel available for scheduling is composed of a group of sub-carriers at the physical layer. The permutation of sub-carriers to form a sub-channel has a direct impact on the amount of channel diversity that can be leveraged through scheduling. Selecting sub-carriers in a distributed manner from the entire spectrum averages out and removes channel diversity (referred to as distributed permutation, DP), while grouping them contiguously retains channel diversity (referred to as contiguous permutation, CP).
The multicast rate of transmission on a sub-channel depends on the minimum rate supported by the users in the session on that sub-channel. Note that the channel quality (and hence the rate) of a user varies across sub-channels in CP, while it remains the same in DP. Hence, if the net (sum) rate of a user on a given set of sub-channels is the same in both DP and CP, then the channel diversity in CP will contribute to degrade the multicast rate on individual sub-channels, reducing its performance below that of DP. Thus, the inherent diversity across sub-channels poses a challenge for multicasting in CP.